1. Field of the Invention
This invention relates to high-resolution display apparatus utilizing a cathode ray tube (CRT) for presenting color graphics. More particularly, this invention relates to such apparatus wherein the CRT control data is stored in digital format and is converted to analog format for controlling the CRT electron beam guns.
2. Description of the Prior Art
Systems for producing high-resolution displays on CRTs, such as for computer graphics displays, commonly develop the picture control data in digital format. Such data is stored in a so-called frame buffer comprising a large number of random-access-memory devices (Video RAMs) arranged to provide a large number of storage banks for respective CRT pixel signals. The frame buffer is controlled by a graphics processor which directs the transfer of data to and from the frame buffer in properly synchronized fashion. The digital data read-out from the frame buffer is directed to a set of digital-to-analog converters which produce corresponding analog signals for respective color guns of the CRT, thereby to control the intensity of each color (red, green, blue) for each pixel of the CRT display.
One of the problems encountered in achieving high performance in CRT displays is caused by the non-linear "transfer function" between corresponding electrical and optical quantities in both the camera and the receiver. For example, in a typical color receiver, the light-output of each phosphor follows a power-law relationship to the video voltage applied to the grid or cathode of the CRT. Consequently, the human eye's perception of incremental changes in light intensity is non-linear, such that the eye perceives a given increment of intensity signal change as a function of the level of the intensity signal. The red intensity, for example, produced on a monitor screen by a red input value of R.sub.i is: EQU R.sub.m =(R.sub.i).gamma.r
where .gamma..sub.r is normally in the range of 2.3-2.8. If .gamma..sub.r, .gamma..sub.g and .gamma..sub.b (i.e., gamma red, gamma green and gamma blue) are known, then a so-called gamma correction can be applied, as in the following relationship: EQU R.sub.i =k(R.sub.i).sup.1/.gamma. r
Prior art systems have incorporated means to apply such gamma correction. However, in those systems, gamma correction generally was achieved at the price of a reduction in the resolution and dynamic range of the color intensity control signals, thus preventing the achievement of really high-quality high-resolution display systems. Moreover, the gamma correction and associated circuitry was unsuitably organized, e.g., so as to require a plurality of separate interconnected components, excessive IC chip area, and so on. Consequently, there has developed a pressing need for improved CRT control systems which avoid the disadvantages of prior art systems.